Process for manufacturing a DMOS transistor

ABSTRACT

In the processes known so far, for a trench-shaped structure the doping of the side walls is coupled to the doping of the floor region.  
     According to the new process, it is possible in the case of a trench-shaped structure to set the doping of the side walls independently from the doping of the floor region. Furthermore, it is also possible to set different dopings among the side walls. For DMOS transistors, this allows high breakthrough voltages to be generated even with low turn-on resistances, and the space requirement, in particular with regard to driver structures, can be reduced.

BACKGROUND

[0001] 1. Field of the Invention

[0002] The present invention relates to a process for manufacturing a DMOS transistor in accordance with the preamble of Patent claim 1.

DESCRIPTION OF THE RELATED TECHNOLOGY

[0003] Such a process is known from the printed publication U.S. Pat. No. 5,539,238. Here, a DMOS transistor with a deep trench structure is generated, with the doped regions adjoining the side walls and the base region representing the so-called drift zone of the transistor. Due to the partially vertical implementation of the drift zone along the side walls of the trench, the length of the transistor can be reduced. The disadvantage in this process is that for an applied blocking voltage inhomogeneities in the course of the potential occur on the edges of the trench structure, which cause an undesirable reduction in the transistor blocking voltage. Furthermore, the total length of the drift region is not decreased but only subdivided into a vertical and a lateral share, that is, the specific turn-on resistance Rsp=Rdson/region is not decreased, rather the side walls can only be doped insufficiently; and the specific turn-on resistance, and thus the surface area used by the transistor, are increased. A further process is known from the printed publication EP 0 837 509 Al. Here, a self-adjusted drift region is generated in a DMOS transistor below a LOCOS oxide. The disadvantage is that the doping of the drift region is introduced before oxidation and that the share of the doping agent diffusing into the oxide can be determined with some imprecision only. In addition, the high temperature load during oxidation causes a wide distribution of the doping agent, which in turn leads to a higher imprecision in the doping agent concentration. Furthermore, a large silicon thickness is required underneath the oxide in order to increase the blocking voltage by means of the so-called “RESURF” effect. Overall, the process scatterings increase the scatter in the electrical parameters of the transistor. One development aim in the area of DMOS transistors is to manufacture space-saving structures which, for an applied blocking voltage, feature low field strengths, in order to avoid a generation of load carriers that lead to a breakthrough within the component. A further aim in the development of DMOS transistors is to achieve a low specific turn-on resistance, Rsp, in order to reduce the spatial area required by such an integrated circuit in the case of integrated circuits where DMOS transistors take up a significant part of the total chip surface area.

SUMMARY OF THE INVENTION

[0004] The present invention is based on the task to state a process by means of which DMOS transistors can be provided on a compact surface area for high blocking voltages.

[0005] This first above-mentioned task is solved by the characterizing features of Patent claim 1. Advantageous forms of embodiment are the subject of sub-claims.

[0006] In accordance with the above, the invention essentially consists in that a trench-shaped structure is generated in a DMOS transistor where, by selecting the doping agent profile within the region of the trench-shaped structure, a high breakthrough voltage for a low lateral expansion of the DMOS transistor is achieved. To this end, in a semiconductor body of a first conductivity type, which features a surface layer in which a source region and a drain region of a second conductivity type, and a first well region of a first conductivity type that includes the source region, and a second well region of a second conductivity type that includes the drain region, are formed, and where on the surface of a semiconductor body a gate region is formed, which—starting at the source region—extends fully across the well region of the first conductivity type, and which—starting at the surface of the semiconductor body—generates a trench-shaped structure in a part of the surface layer. Furthermore, in the floor region of the trench-shaped structure, a doping of a second conductivity type with a first concentration, and in the source-end side wall of the trench-shaped structure a doping of the second conductivity type with a second concentration, and in the drain-end side wall of the trench-shaped structure a doping of the second conductivity type with a third concentration, are generated.

[0007] An essential advantage of the new process is that, due to the different concentration of the doping agent in the source-end side wall compared to the drain-end side wall in connection with the doping agent concentration in the floor region of the trench-shaped structure, which together define the drift range of the transistor, a simple optimization within a parameter field essentially determined by the specific turn-on resistance Rsp, breakthrough voltage Vbreak, and the size and shape of the SOA (safe-operation-area), can be carried out. In particular for driver structures it thus becomes possible to generate transistors with a compact total area. Furthermore, the RESURF effect can be optimized particularly advantageously with regard to: starting point, using different doping agent concentrations; strength, by means of an adjustable vertical distribution of the potential gradient for the applied blocking voltage. As doping is effected only after silicon etching with a low implantation energy, and no thick LOCOS-oxide with a high temperature load is generated next, spatially highly doped regions can be generated along a short vertical route underneath the floor of the trench-shaped structure; these spatially highly doped regions form a buried current path with low resistance. As the doping of the floor—by means of the RESURF-effect in connection with the doping course in the source-end side wall—has an essential influence on the breakthrough voltage in a blocking as well as in a switched on condition, whilst the drain-end doping agent course has an essential influence on the turn-on resistance Rdson, an adaptation of the doping profiles along the trench-shaped structure to the electrical requirements is particularly advantageous. Furthermore, the space used by the transistors is reduced as, due to the self-adjustment in connection with a simultaneously reduced temperature load compared with a LOCOS oxidation, the process scattering of the doping agent profiles introduced into the trench-shaped structure are reduced.

[0008] In a further embodiment of the process, it is advantageous to expand the region of the second well in the direction of the source and to generate the trench-shaped structure partially or wholly within the region of the second well. The further the second well extends in the direction of the source, the more the specific turn-on resistance Rsp is reduced, as the floor region of the trench-shaped structure and the second well feature the same doping polarity. At the same time the transistor features a high breakthrough voltage as the concentration of the dopings for the first and second wells is significantly lower than the concentration of the source and drain regions. Furthermore, both wells can be produced by means of LOCOS oxidation in a single mask step and with self-adjustment. It is advantageous here, to drive in the first well more deeply and for a greater length in order to generate a RESURF effect underneath the trench-shaped structure by means of a lateral PN junction, which RESURF effect increases the breakthrough voltage.

[0009] In a further embodiment of the process, an extension region is generated underneath the drain-doping region, which extension region completely encloses the drain region, with the doping of the extension region being of the same conductivity type, but featuring a lower concentration than the drain region. In addition to the suppression of a drain-end breakthrough, the reduction of the resistance within the drain-end side wall region reduces the specific turn-on resistance Rsp. The specific turn-on resistance Rsp is reduced particularly strongly if the extension region and/or the drain-end region connect immediately to the drain-end side wall of the trench-shaped structure. Investigations carried out by the applicant have shown that in the breakthrough region of the transistor, by means of a distance between the drain-end side wall of the trench-shaped structure and the extension region and/or the drain-end—which is preferably between 0.5 μm and 4.0 μm—, a balancing can be achieved. Here, by means of the additional drain-end resistance causing a voltage drop, a local excessive increase in the current density is suppressed. In particular in connection with a clamp controller, advantageous ESD protective structures can be produced by means of such transistors.

[0010] In a further development of the process, in the side walls and in the floor region of the trench-shaped structure, a higher doping agent concentration than in the body region is generated, in order to increase the maximum blocking voltage by means of the RESURF effect, and also to reduce the specific turn-on resistance Rsp.

[0011] In another development of the process, the same doping agent concentration is generated in the source-end side wall as well as in the drain-end side wall of the trench-shaped structure. This simplifies the introduction of the doping agent and increases the specific turn-on resistance Rsp by only a minor degree, as the individual doping agent concentrations add up, if the extension region and the drain region start immediately on the side wall of the trench-shaped structure, and if the introduction depth of the drain-end doping lies within the range of the trench-shaped structure. Furthermore, it is advantageous—in particular with regard to deep trench-shaped structures—to generate a higher doping agent concentration in the drain-end side wall than in the source-end side wall, in order to obtain a lower turn-on resistance Rsp.

[0012] Investigations by the applicant with regard to different doping agent concentrations for wall and floor have shown that it is advantageous, if the aspect ratio of the trench-shaped structure is above 0.5, and features a trench-shaped structure with a width in a range between 0.5 μm and 4.0 μm. In order to suppress excessive field strength increases on the edges of the trench-shaped structure, it is advantageous to generate inclined side walls, that is, the trench-shaped structure features a narrower width in the floor region than on the surface. The generation of the trench-shaped structure can be carried out by means of a dry etching process such as, for example, a shallow trench process (STI), and filled up with an isolating material such as a CVD-oxide or nitride, and planarized by the use of a CMP process.

[0013] In another development of the process, the trench-shaped structure is generated by means of a V trench etching process and filled up in a following LOCOS oxidation which, due to the lower temperature load, is preferably produced by high pressure oxidation. The doping of the side walls and the floor is carried out prior to the LOCOS oxidation, with the infed dosage of doping agents being increased by that proportion which is diffused into the oxide during oxidation.

[0014] In a further development of the process, the DMOS transistor is generated in the surface layer of a waver with an isolating intermediate layer. It is advantageous here, if the thickness of the residual surface layer underneath the trench-shaped structure is between one half and a factor 5 of the depth of the trench-shaped structure. It is furthermore advantageous, if the drain-end region and/or extension region, and the two well regions as well as the source region connect immediately to the isolating intermediate layer in order to suppress the parasitical capacities. A further advantage is that the required layer thickness of the surface layer is within a range of just a few um, as the formation of a highly doped buried channel connecting to the floor of the trench-shaped structure features a low vertical extension only.

[0015] Investigations by the applicant have shown that the DMOS transistors produced by the process in accordance with the invention, in particular when using a silicon wafer with an isolating intermediate layer, are especially suitable for the manufacture of high blocking integrated circuits featuring an output driver for driving inductive loads.

BRIEF DESCRIPTION OF THE FIGURES

[0016] In the following, the process according to the invention is to be explained in more detail by means of embodiment examples and in connection with several block diagrams. The figures below show:

[0017]FIG. 1 a cross-section of a DMOS transistor with a trench-shaped structure in the drift region, and

[0018]FIG. 2a a cross-section of the doping layers for a DMOS transistor between the gate and drain regions, and

[0019]FIG. 2b a potential course for the DMOS transistor from FIG. 2a, with a blocking voltage applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Table 1 shows a process sequence for manufacturing DMOS transistors, into which—based on the BCDMOS process sequence known from the current state of the art—the process steps 4 to 8 are additionally inserted. This generates a trench-shaped structure between the source and drain regions of N- or P-DMOS transistors, with a freely selectable doping in the respective side wall and a separately selectable doping in the floor region. With such process sequences, it is possible to produce at the same time N- and P-DMOS transistors, bipolar and complementary MOS transistors for an integrated circuit.

[0021] Table 1: Standard DMOS module:

[0022] 1. Material start: Silicon wafer with an isolating intermediate layer

[0023] 2. Formation of the P-well (mask step)

[0024] 3. Formation of the N-well (self-adjusted to P-well)

[0025] 4. FORMATION OF THE WINDOW FOR A TRENCH-SHAPED STRUCTURE (MASK STEP)

[0026] 5. TILT-IMPLANTATION (FOR SIDE WALL)

[0027] 6. FORMATION OF A TRENCH-SHAPED STRUCTURE BY MEANS OF ANISOTROPE SILICON ETCHING (STI PROCESS) (MASK STEP)

[0028] 7. FORMATION OF A PROTECTIVE LAYER BY OXIDATION/DIFFUSION OF THE IMPLANTED DOPING

[0029] 8. IMPLANTATION VERTICAL AND TILT (FOR FLOOR- AND DRAIN-END SIDE WALL) (MASK STEP)

[0030] 9. Formation of component boxes by means of a deep trench process (mask step)

[0031] 10. Filling of the trench windows, or the trench-shaped structure, with CVD-OXIDE

[0032] 11. CMP-planarization

[0033] 12. Extension-implantation (mask step)

[0034] 13. Threshold voltage implantation (mask step)

[0035] 14. Gate oxidation

[0036] 15. Gate poly separation and structuring (mask step)

[0037] 16. LDD implantation (mask step)

[0038] 17. Source/Drain implantation (mask step)

[0039] 18. BPSG-separation

[0040] 19. Etching of contact windows (mask step)

[0041] 20. Metal 1 (mask step)

[0042] 21. Via-etching (mask step)

[0043] 22. Metal 2 (mask step)

[0044] The starting point for the manufacture of a DMOS transistor in accordance with the process sequence shown is a silicon wafer with an isolating intermediate layer (SOI wafer). Following the definition of the wells, a window is defined by means of mask step 4, through which window the doping for the side walls of the structure to be generated is implanted by means of a subsequent step 5. In the following step 6, a trench-shaped structure is produced by silicon etching, and then, in a follow-on step 7, the side walls are lined with a protective layer. In a follow-on step 8, a two-stage implantation for doping the floor- and the drain-end side wall is carried out. As the process steps 4 to 8 are carried out with a single mask, the implantations thus introduced are self-adjusted. The filling of the trench-shaped structure is implemented jointly with the filling of the trench structures. In further process steps, the gate regions and the source/drain regions are defined and connected to the printed circuit path system by means of a contact window process. Due to the use of an SOI wafer with a trench isolation, the transistors produced are located in individual component boxes isolated from each other.

[0045] In the following, FIG. 1 is explained; this shows a block diagram with a cross-section of an N-DMOS transistor 100 with a trench-shaped structure. The manufacture of an N-DMOS transistor 100 is implemented by means of a process sequence (not shown here) within a semiconductor body 5 featuring an isolating intermediate layer 4. To this end, in a first process step, a P well region 20 and an N well region 19, with self-adjustment, are produced in the semiconductor body 5—for example by means of LOCOS oxidation. The position of the PN junction formed by the two wells can be shifted along a line X1 by changing the position and size of the P well mask region. In a subsequent process step, within a protective layer consisting for example of a nitride and/or oxide, a mask step is used to produce a window for the introduction of a doping agent with negative polarity such as arsenic or phosphor. In order to increase the drain-end concentration of the doping agent, the implantation is made at a tilt angle of 60 degrees, for example. In a following process step, a trench-shaped structure is produced by means of anisotrope silicon etching, for example by means of STI etching, and the doping agent in the floor region is removed completely. In a following process step, a scattering oxide is produced whose thickness is sufficient to suppress effectively the doping agent infeed by means of a second implantation in the floor region of the trench-shaped structure. As the side walls are only slightly inclined, oxide thicknesses within the range of just a few 100 A suffice for this purpose. Due to the oxidation step, the doping agent remaining from the first implantation step is simultaneously diffused, with a first region 40 with a first concentration forming at the source end, and a second region 60 with a second concentration forming at the drain end. In a following process step, in a second implantation step which is carried out in two stages, a doping agent with a negative polarity is introduced. Here, in the first stage, a part of the total dosage is introduced vertically, that is, only into a floor region 50, and in the second stage the remaining dosage is introduced at a tilt angle of 60 degrees, for example, so that the concentration levels in the drain-end region of the floor 50 and in the drain-end side wall 60 are increased even further. In summary, following the two implantation steps, the region 60 features a high concentration, the region 50 a medium concentration, and the region 40 a lower concentration, of a doping agent with negative polarity. In a following process step, the trench-shaped structure is filled with an isolating material, with a CVD oxide 65 for example, and the surface of the trench-shaped structure is planarized by a CMP step. In several following process steps that are immanent in known MOS process architectures, a gate connection G with a gate oxide 30 and a poly-silicon layer 35 is produced. Furthermore, in subsequent process steps that are also known, a source connection S with a highly doped region 10 and a drain connection D with a highly doped region 80, which feature a negative polarity, as well as a body connection B with a highly doped region 15 featuring a positive polarity, is produced. Also, below the drain connection D, an extension region 70 with negative polarity is produced, whose concentration is lower than the concentration in the region 80. Furthermore, the extension region 70 and the drain region 80 connect immediately to the drain-end side wall of the trench-shaped structure so that the concentrations of regions 60, 70, 80 add up along the side wall. Additionally, the region of the source connection 10 is enclosed by the P well region 20, with the P well connecting immediately to the N well in a lateral direction. Also, the P well region 20 and the N well region 19 border directly onto the isolating intermediate layer 4.

[0046] The particular advantage in the process sequence shown is that it can be used to produce jointly N-DMOS as well as P-DMOS transistors, with the temperature load during the production process being significantly reduced due to the trench-shaped structure being produced by dry etching. In this way, spatially delimited regions with differently high dopings can be produced along the trench-shaped structure parts, by means of which spatially delimited regions it is then easy to optimize the electrical parameters of the DMOS transistor. In particular, with the low specific turn-on resistances Rsp and the simultaneous high blocking voltages, large current carrying capacities on a compact surface area can be produced, as a.o. the voltage drop within the drift range of the transistor reduces. Furthermore, by adding epitaxy layers and/or buried layers, it is possible to isolate the DMOS transistors against each other using just a few additional process steps. Furthermore, due to the formation of the N well in the drift range region of the transistor, the saturation current Idsat is increased. Additionally, the blocking voltage increases as the N well 19 region, in comparison to the extension region 70, features a reduced doping agent concentration.

[0047]FIG. 2a shows a cross-section view of the doping layers of the part between gate region 35 and drain region 80 of the N-DMOS transistor 100 from FIG. 1. The N-DMOS 100 is produced by a process sequence as explained in connection with the drawings of FIG. 1, with—in extension—a passivizing layer made of oxide 105 being applied to the surface of the semiconductor structure. Furthermore, the layer 105 features a window for the gate connection G as well as a window for the drain connection D, which are both filled by a metal. Moreover, in the regions 19, 20, 35, 50, 70, and 80 the polarity of the doping agent is represented by the direction of the hatching. Here, those regions which feature a negative polarity are hatched by lines from the top left to the bottom right, and those regions which feature a positive polarity are hatched by lines from the top right to the bottom left. Furthermore, the level of doping agent concentration in the respective region is represented by the density of the hatching. Furthermore, the N well 19 region includes the extension region 70, with the N well 19 region and the region of P well 20 featuring comparable doping agent concentration levels. Furthermore, underneath the trench-shaped structure between the P well region 20 and the N well region 19, a PN junction is produced which features a lateral component. To this end, for example, the P well is produced before the N well.

[0048]FIG. 2b shows the potential course for the transistor illustrated in FIG. 2a, with a blocking voltage applied, just ahead of the breakthrough. Here, the family of the individual potential lines represents the potential course between the channel region underneath the gate oxide 30 and the region of drain 70, 80, with the location of the highest field strength being represented by the location with the highest density of potential lines. Here it becomes clear that the transistor features an even distribution of the potential lines within the drift range, with the RESURF effect being reinforced in particular by the lateral formation of the PN junction below the trench-shaped structure between both wells. In particular, the region of the source-end side wall in the trench-shaped structure is preferably cleared by the formation of a spatial load zone, so that the RESURF effect already starts at low voltages and field strength peaks are avoided. Due to the even distribution of the potential lines within the drift range, high blocking voltages are achieved whose height is essentially determined by the concentrations of the doping agents in the well regions 19 and 20. Furthermore, due to the high concentration of the doping agent—in particular in the drain-end region of the trench-shaped structure—in connection with the N well, the current carrying capacity Idsat of the DMOS transistor is increased and the specific turn-on resistance is reduced. This reduces the region requirement in particular for driver applications with high currents.

[0049] A further advantage is the easy transfer of the new process to wafers featuring an isolating intermediate layer such as SOI wafers, for example. The vertically delimited highly doped regions below the trench-shaped structure reduce the thickness of the surface layer, with the formation of a lateral PN junction below the trench-shaped structure causing high blocking voltages to be achieved—even in a turned-on condition. Furthermore, the underlying isolating intermediate layer—which in general features a higher dielectric constant than silicon—reinforces the RESURF effect by means of an immanent bundling of the potential lines and increases the blocking voltages between drain and source by means of the low field strength within the drift range. In particular, the layer thickness for the surface layer located on the isolating intermediate layer can be kept low, and further component types such as, for example, bipolar and MOS transistors can be integrated together with the DMOS transistors on a single wafer—at low cost and requiring just a few additional process steps. Additionally, in particular due to the low thickness of the surface layer, the parasitical capacities can be suppressed by extending a part of the doping regions—such as, for example, the P well, the N well, and the extension region—right up to the isolating intermediate layer. 

What is claimed is:
 1. Process for manufacturing a DMOS transistor (100) with a semiconductor body (5), which features a surface layer with a source region (10) and a drain region (80) of a second conductivity type and a first well region (20) of a first conductivity type, enclosing the source region (10), and a second well region (19) of a second conductivity type, enclosing the drain region, and with a gate region (35) being formed on the surface of the surface layer of the semiconductor body (5), which gate region—starting at the source region (10)—extends across the first well region (20), wherein starting on the surface of the semiconductor body (5) in the surface layer, a trench-shaped structure is produced, and wherein in the floor region (50) of the trench-shaped structure a doping of the second conductivity type is produced with a first concentration value, and wherein in the source-end side wall (40) of the trench-shaped structure a doping of the second conductivity type is produced with a second concentration value, and wherein in the drain-end side wall (60) of the trench-shaped structure a doping of the second conductivity type is produced with a third concentration value.
 2. Process according to claim 1 wherein the trench-shaped structure is produced partially or wholly within the second well region (19).
 3. Process according to claim 1 wherein, below the drain region (80), an extension region (70) of a second conductivity type is produced which encloses the drain region (80).
 4. Process according to claim 2 wherein, below the drain region (80), an extension region (70) of a second conductivity type is produced which encloses the drain region (80).
 5. Process according to claim 1 wherein the drain region (80) is produced immediately adjacent to a side wall of the trench-shaped structure.
 6. Process according to claim 4 wherein the drain region (80) is produced immediately adjacent to a side wall of the trench-shaped structure.
 7. Process according to claim 4 wherein the extension region (70) is produced immediately adjacent to a side wall of the trench-shaped structure.
 8. Process according to claim 4 wherein the drain region (80) and the extension region (70) are produced immediately adjacent to a side wall of the trench-shaped structure.
 9. Process according to claim 4 wherein a distance between the side wall of the trench-shaped structure and the extension region (70) is produced which is preferably between 0.5 μm and 4.0 μm.
 10. Process according to claim 1 wherein a distance between the side wall of the trench-shaped structure and the drain region (80) is produced which is preferably between 0.5 μm and 4.0 μm.
 11. Process according to claim 4 wherein a distance between the side wall of the trench-shaped structure and the drain region (80) is produced which is preferably between 0.5 μm and 4.0 μm.
 12. Process according to claim 4 wherein a distance between the side wall of the trench-shaped structure on the one hand and the extension region (70), and the drain region (80) on the other hand, is produced which is preferably between 0.5 μm and 4.0 μm.
 13. Process according to claim 1 wherein—in the side walls and in the floor region (50) of the trench-shaped structure—a doping is produced which features a higher concentration level than in the first well region (20).
 14. Process according to claim 8 wherein—in the side walls and in the floor region (50) of the trench-shaped structure—a doping is produced which features a higher concentration level than in the semiconductor body (5) and/or the first well region (20).
 15. Process according to claim 12 wherein—in the side walls and in the floor region (50) of the trench-shaped structure—a doping is produced which features a higher concentration level than in the semiconductor body and/or the first well region (20).
 16. Process according to claim 1 wherein a trench-shaped structure is produced, with the second and third doping agent concentration values being equal.
 17. Process according to claim 8 wherein a trench-shaped structure is produced, with the second and third doping agent concentration values being equal.
 18. Process according to claim 12 wherein a trench-shaped structure is produced, with the second and third doping agent concentration values being equal.
 19. Process according to claim 1 wherein a trench-shaped structure is produced, with the second doping agent concentration value being lower than the third doping agent concentration value.
 20. Process according to claim 8 wherein a trench-shaped structure is produced, with the second doping agent concentration value being lower than the third doping agent concentration value.
 21. Process according to claim 12 wherein a trench-shaped structure is produced, with the second doping agent concentration value being lower than the third doping agent concentration value.
 22. Process according to claim 1 wherein a trench-shaped structure is produced whose aspect ratio is above 0.5 and whose width is within a range between 0.5 μm and 4.0 μm.
 23. Process according to claim 1 wherein a trench-shaped structure is produced, which features a narrower width in the floor region (50) than on the surface.
 24. Process according to claim 1 wherein the trench-shaped structure is produced by STI etching and filled with an isolating material, preferably by means of an oxide.
 25. Process according to claim 1 wherein the trench-shaped structure is produced by means of a LOCOS oxidation.
 26. Process according to claim 1 wherein the semiconductor body features an isolating intermediate layer (4) below the surface layer, and the DMOS transistor is produced in the surface layer.
 27. Process according to claim 26 wherein the thickness of the surface layer underneath the trench-shaped structure is within a range between one half and a factor 5 of the depth of the trench-shaped structure.
 28. Process according to claim 1 wherein the thickness of the surface layer underneath the trench-shaped structure is within a range between one half and a factor 5 of the depth of the trench-shaped structure.
 29. Process according to claim 26 wherein the drain region (80) and/or the extension region (70) as well as the two well regions (19, 20), and the source region (10), are produced immediately adjacent to the isolating intermediate layer (4). 